text: Stormwater garden design encompasses a wide range of features based on the principles of bioretention. These facilities are then organized into a sequence and incorporated into the landscape in the order that rainfall moves from buildings and permeable surfaces to gardens, and eventually, to bodies of water. A rain garden requires an area where water can collect and infiltrate, and plants can maintain infiltration rates, diverse microorganism communities, and water storage capacity. Because infiltration systems manage storm water quantity by reducing storm water runoff volumes and peak flows, rain garden design must begin with a site analysis and assessment of the rainfall loads on the proposed bioretention system. This will lead to different knowledge about each site, which will affect the choice of plantings and substrate systems. At a minimum, rain gardens should be designed for the peak runoff rate during the most severe expected storm. The load applied on the system will then determine the optimal design flow rate.
  Existing gardens can be adapted to perform like rain gardens by adjusting the landscape so that downspouts and paved surfaces drain into existing planting areas. Even though existing gardens have loose soil and well-established plants, they may need to be augmented in size and/or with additional, diverse plantings to support a higher infiltration capacity. Also, many plants do not tolerate saturated roots for long and will not be able to handle the increased flow of water. Rain garden plant species should be selected to match the site conditions after the required location and storage capacity of the bioretention area are determined. In addition to mitigating urban runoff, the rain garden may contribute to urban habitats for native butterflies, birds, and beneficial insects.
  Rain gardens are at times confused with bioswales. Swales slope to a destination, while rain gardens are level; however, a bioswale may end with a rain garden as a part of a larger stormwater management system. Drainage ditches may be handled like bioswales and even include rain gardens in series, saving time and money on maintenance. Part of a garden that nearly always has standing water is a water garden, wetland, or pond, and not a rain garden. Rain gardens also differ from retention basins, where the water will infiltrate the ground at a much slower rate, within a day or two.